The hair follicle undergoes a cycle of hair growth (anagen) followed by regression (catagen), and quiescence (telogen) until a new hair shaft is generated in the existing follicle during the subsequent anagen phase. Hardy et al. (1992) Trends in Genetics 8:55-61. The hair shaft is derived from the epithelial matrix cells at the base of the follicle, but a cluster of dermal cells ensheathed by the matrix cells, known as the dermal papilla (DP), is thought to supply inductive signals required for hair outgrowth. Reciprocal signaling from the epidermis is required for the formation of the dermal papilla and may also explain the coordinated morphological changes in epithelial and dermal components of the follicle observed during the hair cycle.